The present invention relates to a concentrated-constant, non-reciprocal device for use in a microwave band comprising a ferrimagnetic body, particularly to a miniaturized, broad-band concentrated-constant circulator/isolator.
According to recent miniaturization of semiconductor elements such as ICs, transistors, etc., passive elements such as laminated chip capacitors, laminated chip inductors, chip resisters, etc., microwave devices surface-mounted with such elements are rapidly miniaturized and reduced in thickness. In such circumstances, concentrated-constant, non-reciprocal devices extremely important for microwave devices are also required to be miniaturized and reduced in thickness.
As a conventional concentrated-constant, non-reciprocal device, there is, for instance, a concentrated-constant isolator having three terminals, one of which is connected to a resister Ro. In the concentrated-constant isolator, a signal does not substantially attenuate in a transmission direction, though it is extremely decreased in the opposite direction. Thus, the concentrated-constant isolator is used for mobile communications equipments such as cellular phones.
FIGS. 22(a) and (b) schematically show the structure of such a concentrated-constant isolator. This concentrated-constant isolator has a structure in which three sets of center conductors 1a, 1b, 1c are intertwined on a ferrimagnetic body 2. One side of the center conductors functions as input/output terminals {circle around (1)}, {circle around (2)}, {circle around (3)}, and the other is connected to a common part 3 (ground conductor in this example), with crossing portions of the center conductors free from short-circuiting by insulating sheets 4. Capacitors C connected between the input/output terminals {circle around (1)}, {circle around (2)}, {circle around (3)} and the common part 3 (ground conductor) determines an operating frequency of the circulator. By applying an external magnetic field 5 to the ferrimagnetic body 2, the circulator is operated at a desired impedance Z0. Also, to function as an isolator, a resister R0 is connected between the input/output terminal {circle around (3)} and the common part (ground conductor) 3.
FIG. 23 shows an equivalent circuit of the above concentrated-constant isolator, in which each terminal of an ideal circulator having three input/output terminals {circle around (1)}, {circle around (2)} and {circle around (3)} is connected to an LC parallel resonance circuit. In the figure, the terminal {circle around (1)} is an input terminal; the terminal {circle around (2)} is an output terminal; and the terminal {circle around (3)} is connected to a resister R0 having the same resistance as that of the impedance Z0. In FIGS. 22 and 23, C is capacitance, and L is inductance in the ferrimagnetic body 2, around which the center conductors are wound. The inductance L changes depending on the external magnetic field 5. In the circulator adjusted to match the external impedance Z0, the LC resonance circuit is resonant at a center frequency f0, while input impedance Z0 is zero when a matching load is connected to each terminal.
As another example of the concentrated-constant, non-reciprocal devices, there is a two-terminal, concentrated-constant isolator schematically shown in FIGS. 24(a) and (b). In the two-terminal, concentrated-constant isolator, two sets of center conductors 1a, 1b are disposed substantially in a perpendicularly crossing manner on the ferrimagnetic body 2. One end of each center conductor is an input/output terminal {circle around (1)}, {circle around (2)}, and the other end is connected to a common part 3 (ground conductor in this example), with crossing portions of the center conductors free from short-circuiting by insulating sheets 4. Capacitors C connected between the input/output terminals {circle around (1)}, {circle around (2)}, and the common part 3 (ground conductor) determines an operating frequency of the circulator. By applying an external magnetic field 5 to the ferrimagnetic body 2, the circulator is operated at a desired impedance Z0. Also, in the case of transmission in the opposite direction, a resister R0 is connected between the terminals {circle around (1)} and {circle around (2)} for energy absorption.
FIG. 25 shows an equivalent circuit of the above two-terminal, concentrated-constant isolator, in which each terminal of an ideal non-reciprocal phase shifter having two input/output terminals {circle around (1)} and {circle around (2)} is connected to an LC parallel resonance circuit. In the figure, the terminal {circle around (1)} is an input terminal; the terminal {circle around (2)} is an output terminal; and an ideal non-reciprocal phase shifter is connected in parallel to a resister R0 for absorbing energy at the time of transmission in an opposite direction. This ideal non-reciprocal phase shifter makes the phase advance by 2xcfx80 when the microwave proceeds in a forward direction, while it makes the phase advance by xcfx80 when the microwave proceeds in the opposite direction. In FIGS. 24 and 25, C is capacitance, and L is inductance in the ferrimagnetic body, around which the center conductors are wound. The inductance L changes depending on the external magnetic field 5. In the isolator adjusted to match the external impedance Z0, the LC resonance circuit is resonant at a center frequency f0, while input impedance Z0 is zero when each terminal is connected to a matching load.
In general, the sizes of the concentrated-constant, non-reciprocal devices in the above two examples are determined by the sizes of ferrimagnetic bodies (garnet) 2 included therein. The optimum size of the ferrimagnetic body is about xe2x85x9 of a wavelength xcexg of an electromagnetic wave proceeding in the ferrimagnetic body, at which the ferrimagnetic body is operated in a magnetic field providing the smallest insertion loss. However, because extreme miniaturization makes the ferrimagnetic body 2 much smaller than the optimum size, a magnetic field has to be large, resulting in drastically narrowed bandwidth.
According to recent increase in the number of users, necessity is generated to cover a wide bandwidth by a single cellular phone. Particularly in a cellular phone usable in a relatively low bandwidth such as 800 MHz, miniaturized, wide-band concentrated-constant circulator/isolators are strongly desired. However, because miniaturization contradicts with the relative bandwidth as described above, the miniaturization of the concentrated-constant, non-reciprocal device simply by reducing the size of the ferrimagnetic body leads to the problem that it fails to cover all the bandwidth necessary for cellular phones.
Accordingly, an object of the present invention is to overcome the above problems in the prior art technologies, thereby providing a miniaturized, wide-band, concentrated-constant, non-reciprocal device, for instance, a circulator or an isolator.
The concentrated-constant, non-reciprocal device of the present invention comprises a permanent magnet for applying a DC magnetic field to a ferrimagnetic body; an assembly comprising a plurality of center conductors wound around or at least partly embedded in the ferrimagnetic body, each center conductor having one end as a common terminal and the other end as a first input/output terminal; a plurality of second input/output terminals; a plurality of impedance-converting circuits each connected between the second input/output terminal and the first input/output terminal; and a plurality of load capacitors each connected between the first input/output terminal and the common terminal; wherein the input impedance Zi of the first input/output terminals and the external impedance Z0 connected to the second input/output terminals meet the relation of 0.05xe2x89xa6Zi/Z0xe2x89xa60.9 at an operating center frequency thereof.
The impedance-converting circuit preferably comprises inductance connected between each first input/output terminal and each second input/output terminal, and electrostatic capacitance connected between each second input/output terminal and a ground conductor. The impedance-converting circuit preferably has electrostatic capacitance smaller than the load capacitance. The inductance of the impedance-converting circuit is preferably provided by a distributed constant line.
The number of the second input/output terminals is preferably three, a resister being connected between one of the second input/output terminals and the ground conductor or the common terminal, whereby the concentrated-constant, non-reciprocal device is operated as an isolator. Also, a resister may be connected between one of the three first input/output terminals and the ground conductor or the common terminal, whereby the concentrated-constant, non-reciprocal device is operated as an isolator.
The inductance and electrostatic capacitance of the impedance-converting circuit and the load capacitance are preferably formed in an integral structure. The integral structure is preferably a ceramic laminate produced by laminating a plurality of green ceramic sheets printed with electrodes, and sintering the resultant laminate at a temperature of 800-1100xc2x0 C.
The integral structure may also be produced by forming at least inductance and electrostatic capacitance of the impedance-converting circuits and the load capacitance connected between the first input/output terminals and the common terminal on the same insulating substrate by a thin-film-forming method.